Compensators comprising retardation films with positive and negative birefringence for use in liquid crystal displays (LCD) are known in prior art.
For example, in LCDs of the vertically aligned (VA) mode it is necessary to compensate the undriven homoeotropic state. Transmission type, VA-mode LCD's can be considerd as controllable retardation layers sandwiched between crossed polarizers. FIG. 1A depicts a typical VA-LCD comprising two polarizers 11 and 12 with crossed optical axes sandwiching a liquid crystal (LC) cell 13 with an LC medium between two substrates 14 and 15 that are covered by transparent electrode layers. The LC medium has a negative dielectric anisotropy Δ∈ and exhibits homeotropic orientation in the undriven state, i.e., the LC molecules 16 are oriented substantially perpendicular to the substrates, which is usually achieved by alignment layers provided on the surfaces of the substrates 14 and 15 that are in contact wth the LC medium. In multidomain (MVA) LCDs the LC cell is additionally divided into multilple, typically four, perpendicular domains. When applying a voltage to the electrodes, the LC molecules in the LC medium will orient sustantially parallel to the substrates due to the negative Δ∈. In case of MVA displays the director, i.e., the preferred direction of the long molecular axes of the LC molecules, will additionally be oriented into different directions in the domains, which provides symmetrical viewing angle characteristics and good color performance. The formation of multiple domains in MVA-LCDs can be achieved by various methods that are known in the art, like for example, by special surface treatment of the alignment layers, by applying slots into the electrodes, or by adding polymeric material into the LC cell.
On axis, the crossed polarizers 11, 12 provide a dark state when the cell is un-driven in the vertically aligned, homeotropic state as shown in FIG. 1A. Off axis, two effects result in light leakage which causes reduced contrast ratio and color washout. Firstly, the homeotropically oriented LC retards the polarized light off axis as shown in FIG. 1B, resulting in a non-linear polarization state. Secondly, the transmission axes of the polarizers, which are orthgonal when viewed on-axis as shown in FIG. 2A, are no longer orthogonal when viewed away from either of the transmission directions as shown in FIG. 2B. The dark state light leakage, as shown in the iso-luminance plot of the display depicted in FIG. 3A, leads to reduced contrast in the 45° directions, as shown in the iso-contrast plot of the display depicted in FIG. 3B.
Compensated VA-LCDs have been reported in prior art. Thus, reduction of light leakage through crossed polarizers is well documented and can be achieved using either biaxial or uniaxial films, as reported in Ishinabe, T. Miyashita, T. Uchida and T. Fujimura, Y, “A wide viewing angle polarizer and a quarter wave plate with a wide wavelength range for extremely high quality LCDs”. Proceedings of the AD/IDW 2001, p485.
Retardation films with positive or negative birefringence are hereinafter referred to as ‘positive’ or ‘negative’ retardation films, respectively.
A combination of two uniaxial films, a positive A-plate aligned with the adjacent polarizer and a positive C-plate will achieve an acceptable dark state for all viewing angles (the terms “A-plate” and “C-plate” are defined below). The homeotropic LC in the undriven state of the VA cell acts as a positive C retardation plate and thus contributes to the compensation effect. However, in a typical VA cell, e.g., 4 μm thickness, there is excessive retardation resulting in overcompensation. This can be cancelled by the addition of a negative C plate to achieve the optimum dark-state.
U.S. Pat. Nos. 6,281,956 and 6,141,075 disclose a VA-LCD comprising a switchable LC layer sandwiched by a first and a second polarizer, and further comprising a positive and a negative retarder. Both the positive and the negative retarder are situated between the display cell and the first polarizer or between the display cell and the second polarizer in order to improve viewing angle and contrast of the display. Further disclosed is a display where the pair of positive and negative retarder is replaced by a biaxial retardation film. For the retardation film it is suggested to use a uniaxially or biaxially stretched plastic film like, for example, a conventional triacetat cellulose (TAC) film, which is typically used in standard displays as protective film covering the polarizers.
However, stretched plastic films like TAC only have very small bireferingence. As the optical retardation is given by the product of the birefringence Δn and the film thickness d, a high film thickness of typically several tens or hundreds of microns is required to provide sufficient retardation values, which is a disadvantage for use in flat panel displays. Furthermore, biaxial films are typically manufactured by a process of side-stretching extruded or cast plastics which is difficult to control.
It was also suggested in prior art to use positive and negative retarders comprising layers of polymerized LC material. Thus, it was shown that a layer of cholesteric LC (CLC) material with a short helical pitch and reflection wavelength peak in the UV range of light (UVCLC) retards visible light with negative C symmetry and can be used as a negative C plate retarder. For example, WO 01/20393 discloses a viewing angle compensator comprising a positive A-plate, a negative C-plate and a positive O-plate retarder. The positive A-plate retarder is, for example, made of polymerized LC material with planar structure. The negative C-plate retarder is, for example, made of polymerized UVCLC material. The positive O-plate retarder is, for example, made of polymerized LC material with tilted or splayed structure. WO 01/20393 further discloses that the compensator can be used in LCDs of the TN or VA mode to improve viewing angle and contrast. As the retardation films comprise LC material having high birefringence, the film thickness can be reduced compared to, e.g., stretched plastic films.
However, the use of a stack of three laminated retarders on each side of the display cell increases the total display thickness and the material, and increases manufacturing costs as the LC film retarders are usually manufactured using two piece-to-piece lamination steps. The A-plate must be aligned perpendicular to the polarizer stretch direction, so it cannot be roll-to-roll laminated. The subsequent UVCLC film, which has no preferred alignment direction, is then laminated onto the A-plate also on a piece-to-piece basis. This increases the number of process steps and the material loss when cutting the LC films to pieces.
An aim of the present invention is to provide a compensator for LC displays, in particular for displays of the VA mode, that does not have the drawbacks of prior art compensators, is easy to manufacture and allows economic fabrication even at large scales. Another aim is to provide a compensator which shows good optical performance when used in LC displays, in particular improved grey level stability at wide viewing angles.
A further aim of this invention is to provide a variety of advantageous uses for the compensator according to this invention.
Further aims of this invention relate to liquid crystal displays, in particular to VA mode LCDs comprising an inventive compensator which show advantageous properties such as good contrast, reduced color shift and wide viewing angles.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The above aims among others can be achieved by providing compensators and liquid crystal displays according to the present invention.